1. Field of the Invention
The present invention relates generally to an element for sensing gases, and more particularly to a gas sensing element which provides significantly improved sensing accuracy and which is suitably used as an instrument for analyzing gases.
2. Discussion of the Prior Art
There has been known a gas sensor which utilizes an ion-conductive solid electrolyte, to measure the concentration (partial pressure) of a given component in an object atmosphere or measurement gas, according to the principle of a concentration cell. For example, an oxygen sensor for determining the concentration or partial pressure of oxygen in a measurement gas, employs zirconia or similar solid electrolyte materials which exhibit relatively high oxygen-ion conductivity at an elevated temperature. For detecting the concentration of water (partial pressure of water vapor), a sensor uses a solid electrolyte material such as cerium or strontium oxides, which are hydrogen-ion conductive at an elevated temperature. Also known is a sensor which uses .beta.-alumina (Na.sub.2 O.11Al.sub.2 O.sub.3) or similar solid electrolyte materials exhibiting sodium-ion conductivity at a high temperature, for measuring the concentration or partial pressure of sulfur dioxide, by utilizing an electrochemical reaction between sodium and sulfur.
Such electrochemically operating gas sensors utilizing a solid electrolyte are used, for example, as oxygen sensors for determining the oxygen concentration in exhaust gases emitted from an internal combustion engine of a motor vehicle, or from industrial furnaces, boilers and similar equipment. The solid electrolyte of such oxygen sensors conventionally used is generally formed as a tubular body which is closed at its one end. In recent years, there has been an increasing trend toward using an elongate planar solid electrolyte body, for easier manufacture, reduced production cost, and increased compactness of the sensors. Such oxygen sensors are formed as a laminar structure having electrodes suitably disposed in contact with the planar solid electrolyte body, such that the electrodes and the solid electrolyte cooperate to constitute an electrochemical cell.
For accurate and reliable operation of the gas sensors even while the temperature of a gas to be measured is relatively low, it is necessary to use a suitable heater for holding the electrodes and solid electrolyte body of the electrochemical cell at a relatively high operating temperature. For example, a heater is disposed in the neighborhood of the electrochemical cell, so as to heat the cell via another solid electrolyte layer or other layer, or disposed in the same plane as, but spaced apart from, the electrodes. Another heater arrangement is disclosed in Japanese Patent Application No. 57-6846 (corresponding to Japanese Laid-Open Publication No. 58-124943 and U.S. Pat. No. 4,510,036), wherein a heater layer is formed over the electrodes of the electrochemical cell via a suitable electrically insulating layer, to heat the electrodes and the underlying portion of the solid electrolyte body. This type of heater arrangement permits reduced size and electric power consumption of the sensor, and relatively rapid heating of the cell.
In the gas sensors discussed above, the concentration of a given component in the object measurement gas is generally determined based on an electromotive force detected by the sensing element, according to the Nernst equation. To assure accurate measurement of the concentration of the desired component, the temperature of the gas sensor must be known. In other words, the sensor must be heated to a known temperature. Conventionally, the temperature of the sensor is considered to be equal to the temperature of the measurement gas, which is detected by a suitable temperature detector such as a thermocouple or thermoelectric thermometer. Such a temperature detector or thermometer is disposed separately from the sensor, more precisely, from its sensing element having the electrochmical cell.
An extensive study and experiment by the inventors revealed a difference between the temerature of the gas sensor and the temperature of the measurement gas. Namely, there may arise an appreciable difference between these temperatures, due to rapid changes in the temperature, flow rate and direction of the measurement gas, and other parameters. The sensor tends to have a temperature gradient in the direction of thickness of the sensor, i.e., in the direction in which the constituent layers are superposed on each other. Therefore, the temperature of the measurement gas detected by a temperature detector positioned near the sensor does not precisely represent the operating temperature of the sensing element of the gas sensor. Accordingly, it is difficult to adequately control the heater based on the temperature measured by such a temperature detector. A research by the inventors showed a temperature difference as large as between +3.5% and -3.5%.